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To find first N prime numbers using Python

 

To find first N prime numbers using Python
trips.dude

26 Oct, 2009 - 09:32 PM
Post #1

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Joined: 26 Oct, 2009
Posts: 1
Hi All,

I am new to the programming world. I was just writing this code to generate N prime numbers. User should input the value for N which is the total number of prime numbers to print out. I have written this code but it doesnt throw the desired output. Instead it prints the prime numbers till the Nth number.
For eg.: User enters the value of N = 7.
Desired output: 2, 3, 5, 7, 11, 13, 19
Actual output: 2, 3, 5, 7

Kindly advise.

CODE
# To find first 'N' prime numbers

i=1
x = int(input("Enter the number:"))

for k in range (1, (x+1), 1):
    c=0;
    for j in range (1, (i+1), 1):
        a = i%j
        if (a==0):
            c = c+1

    if (c==2):
        print (i)
    else:
        k = k-1

    i=i+1
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code_m

RE: To Find First N Prime Numbers Using Python

28 Oct, 2009 - 05:50 AM
Post #2

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Instead of always just printing your result you should append it to a list (named "results" below):
python
result.append(i)


Then break your loop with a len() test:
python
if len(results) == x:
    break


Then print the result outside of your for loops.
python
print " ".join(results)


You might have to fiddle with your loops a little bit, but I'm too tired too be that involved right now tongue.gif

This post has been edited by code_m: 28 Oct, 2009 - 05:52 AM
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noyesa

RE: To Find First N Prime Numbers Using Python

5 Nov, 2009 - 04:19 PM
Post #3

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What your function appears to be doing is finding all prime numbers that are less than N + 1, which doesn't get you what you want.

To produce n number of primes, you have to monitor the number of primes you've actually output. The following is a more readable approach to this same problem that I wrote for a class. It could be more efficient, but I think it does a good job explaining what's going on:

CODE
# Author: Andrew Noyes

def isPrime(num):
    """Checks num for primality. Returns bool."""

    if num == 2:
        return True
    elif num < 2 or not num % 2:    # even numbers > 2 not prime
        return False

        # factor can be no larger than the square root of num
    for i in range(3, int(num ** .5 + 1), 2):
        if not num % i: return False
    return True



def generatePrimes(n):
    """Returns a list of prime numbers with length n"""
    
    primes = [2,]
    noOfPrimes = 1    # cache length of primes for speed
    testNum = 3 # number to test for primality

    while noOfPrimes < n:
        if isPrime(testNum):
            primes.append(testNum)
            noOfPrimes += 1
        testNum += 2

    return primes


You could also use the len() function to check the length of the primes list, but that is much more expensive than keeping a status variable.

This post has been edited by noyesa: 5 Nov, 2009 - 04:25 PM
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